14
S.C. Pan, B. List
loss of stereochemical integrity (Scheme 9, TFA, trifluoroacetic acid).
Measuring NMR spectra and optical rotation of the corresponding HCl
salt allowed us to confirm the expected absolute and relative configuration of the product.
3
Brønsted Acid Catalysis
In the proline-based enamine catalysis, proline actually plays a dual
role. The amino-group of proline acts as Lewis base, whereas the carboxylic group acts as a Brønsted acid (Scheme 10).
The potential of using relatively strong chiral organic Brønsted acids
as catalysts (Specific Brønsted acid catalysis) has been essentially ignored over the last decades. Achiral acids such as p-TsOH have been
used as catalysts for a variety of reactions since a long time, but applications in asymmetric catalysis have been extremely rare. Only very recently, Akiyama et al. (2004, 2005a); also see Akiyama 2004; Akiyama
et al. 2005b) and Terada et al. (Uraguchi and Terada 2004; Uraguchi
Scheme 10. Proline: a bifunctional catalyst
S.C. Pan, B. List
loss of stereochemical integrity (Scheme 9, TFA, trifluoroacetic acid).
Measuring NMR spectra and optical rotation of the corresponding HCl
salt allowed us to confirm the expected absolute and relative configuration of the product.
3
Brønsted Acid Catalysis
In the proline-based enamine catalysis, proline actually plays a dual
role. The amino-group of proline acts as Lewis base, whereas the carboxylic group acts as a Brønsted acid (Scheme 10).
The potential of using relatively strong chiral organic Brønsted acids
as catalysts (Specific Brønsted acid catalysis) has been essentially ignored over the last decades. Achiral acids such as p-TsOH have been
used as catalysts for a variety of reactions since a long time, but applications in asymmetric catalysis have been extremely rare. Only very recently, Akiyama et al. (2004, 2005a); also see Akiyama 2004; Akiyama
et al. 2005b) and Terada et al. (Uraguchi and Terada 2004; Uraguchi
Scheme 10. Proline: a bifunctional catalyst